Power converters are well known and have been used to charge and discharge batteries. Examples of such power converters are disclosed in U.S. Pat. Nos. 4,347,474; 4,549,254; 4,672,303; 4,729,088; and 4,947,311.
FIG. 1 illustrates a battery charge/discharge configuration known in the prior art. The circuit of FIG. 1 includes separate circuits for charging and discharging the battery. The charge and discharge circuits each have their own controller. The interaction of the charge and discharge circuits is controlled by steering logic that is coupled to the charge controller and the discharge controller. A particular problem with the prior art is the need for this additional steering logic to control the operation of the charge and discharge circuits. The steering logic can be very complex and also is an area of possible instability due to the interaction between the charge and discharge circuits.
Another problem with the designs of the prior art is their weight. For converters used in satellites, weight is a primary consideration because of the tremendous launch and payload costs for spacecraft. The battery charge/discharge circuit of FIG. 1 is particularly a problem since separate circuits are required to perform the charging and discharging functions; thus, the power converters add to the weight of the satellite.
A further problem with the converters of the prior art is reliability. This problem is especially troublesome for power converters used in satellites, since the converters are not easily accessible once placed in operation. Thus, separate controlling circuits and complex steering logic are areas were the converters are likely to malfunction. This tends to reduced the reliability of the entire satellite.
U.S. Pat. Nos. 4,736,151 and 4,801,859 issued to Dischner disclose a bidirectional converter. These converters are designed to operate in an electrically-compensated constant speed drive. The control of the converter is not continuous. One switch is held off while the other switch is pulse width modulated. Additionally, the converter of Dischner produces a discontinuity at zero power flow that is detrimental to power charging/discharging systems. Thus, the converters of Dischner would not be generally suitable for satellite applications.
In the design of high power converters, for example 2 kilowatts or above, it is known to divide the power converter into a number, N, of identical sections, each processing a fraction of the total power. These individual sections are often referred to as slices, and the entire converter is then said to be interleaved. The slices are incorporated into an overall architecture which would include a multiple phase oscillator that outputs phases spaced at , a pulse width modulator, and a feedback control amplifier. The feedback control amplifier may contain one or more control loops. Because of the advantages in providing the combination of high performance with high stability margins, dual loop control or current mode control is increasingly becoming a standard design approach.
However, the inventors have determined that providing average current mode control in interleaved converters also requires that some mechanism be provided to prevent an individual slice or section from passing more than its normal share of the total converter power. The consequence of an unbalanced sharing of power is a decrease in efficiency, the possibility of overstressing individual components, and possible inductor saturation.
It should be noted that it is possible for the imbalance to arise from causative factors that are regenerative in nature. An example of such a factor is inductor saturation. That is, if one of the individual inductors is incrementally more susceptible to saturation than its counterparts, it will carry a larger share of the total current. This condition leads to increased saturation and still further current increases until some external factor or event establishes a current limit.